I realize that the R19S is da bomb in light air but how would an R18S +6 compare to an R19S +8? (on a 160). I assume the R18 could go into a higher wind range but would give up a bit in the light stuff.

I also would like to know how it (R18S +6) would compare to my older R13 M +8. Would their benefits Thickness/Softness offset each other and overlap too much?

With SB 160 my best fin was R18S-(+8). I changed R13S+8 because the R18 had more speed at same upwind angle and more easy to go downwind. The R18 is more technical than R13, but if you can handle it, the R18 shows good performance (8-20 knots).
To find the right fin it???s also the matter of personal preference, rider???s weight, style and technique.